Demodulator and method op demodulation



s. BALL NTINE l A ET AL Re. 18,579

Aug' 23 DEMODULATOR AND METHOD OF DEMODULATION' 4 Sheets-Sheet luriginav. Filed June s, 41923 l. Noa-rechi@ "Ihrem/'fige Z RechbryGems/713e' l Y Sito: wu.

s. BALLANTINE ET A!- Re. 18,579

DEMODULATOR AND METHOD OF D'MODULATION 4 Sheets-Sheet 2 AugQzs, 1932.

Original Filed Juno 8,- 1925 Ferro-571km, Z6-f5.

3 and fran-Pic' /zsa s. BALLANTINE ET AL Aug. 23, 1932. K DEMODULATOBAND METHOD oF DEMODULATION Re. 18,579

4 Sheets-Sheet 5 original Filed June a, 1925' Varia/'1bn of fxpanen/'f'nLaw of ies-MM Ferro-J//fcan Fach' /er JWM SWMTMw/ZL,

Jan. 8,*1929.

S. BALLANTINE ET AL DEMODULATOR AND METHOD 0F DEMODULATION originalFiled June 8', 1925 4 Sheets-Shea#I 4 .JWM

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. in which the response is directly proportional to the amplitude oftheim ressed. voltage stimulus.

Reissued Aug. 23, 1.932

UNITED STATES PATENT OFFICEN STUART BALLANTINE AND LEWIS M. HULL, FMOUNTAIN LAKES, NEW JERSEY, AS-

SIGNORS, BY MESNE ASSIGNMENTS, TO RADIO CORPORATION 0F AMERICA, OF NEWYORK,`N. Y., A CORPORATION 0F DELAWARE DEMODLATOB AND. METHOD 0FDEMODULATION Original No. 1,698,668, datedl January 8, 1929, Serial No.644,21-5, il1ed June 8, 1923. Application for reissue illed October 1,1930. Serial No. 485,7 85 i `This invention relates to demodulators foruse in radio receiving circuits, particularly those used for radiotelephony, and to methods of demodulation. I

.An object ofthe invention is to provide a demodulator which is freefrom distortion.

. A further object of the invention is to provide a method ofdemodulation by which the `audio frequency output lcurrent may be madeproportional to the first power of the radio frequency amplitude. Afurther oblect is toprovide a method ofdemodulation y ,which thestimulus-response characteris; ticV of thedemodulator may be adjusted togive a true and undistorted copy of .the

modulating currents operating in the transmitter, and to adjust thetonal characteristics of the sounds produced.

' "The usual system of demodulation is based upon the use 'of a circuitelement or device',l usually called a detector, which possesses anasymmetrically conducting property.

vof the original modulation ofthe si The essential 'property of thedetector, in prior art, is not a .true unilateral conductivity,resulting in simple rectification,` but the property of asymmetricalconductivity corresponding to the curvature of its current-voltagecharacteristic. It has been recognized that the audio-frequency currentobtanedin prior devices is not a true copy al and it can be demonstratedthat the detected audio-frequency current is 4not of a lower order thanthe second power of the impressed signal voltage. According to thepresent invention, however,the old types Iof detectors which rely solelyupon asymmetrical conductivity are L abandoned and we employ instead adetecting element having a certain operatingvrange Thus by means o ourinvention we are able to obtain in response to a modulated radiofrequency signal voltage an audio frequency current Ywhose amplitude-isdi-` frequency response stands in contrast with the. square-law relationcharacteristics of 1on1c tube vand crystal detectors as commonlyemployed and results in a marked improve- F ig. 2 is a dia rammaticrepresentation of static current-vo tage characteristics -of a detectorelement having substantially v constant conductivity over a wide range.

Fig.`3 is a'diagranimatic representation of experimental responsecharacteristics for a detector element aving current-voltagecharacteristics as shown in Fig. 2.'

Fig. 4 is a diagrammatic representation of the variation of the exponentin the law of response for a detector element having a respcnsecharacteristic such as shown in Fig.. 3; an Figs. .5 and 6 are diagramsof apparatus embodying our invention.

The experimental data which is represented in the curves ofFig. 1 wasobtained by measurements taken on an asymmetrical conductor formed vby alight metallic pointcontact upon a crystal of refined silicon'. Thecharacteristic curves were obtained for three different pointsbyimpressin'g a steady but reversible voltage upon .the detector andmeasuring the resulting current in the two directions for direct andreversed voltage. When operated by a radio-frequency signal voltagelover a small range on one of these curves the direct currentandaudio-frequency current components are greaterhthe greater thecurvature of the characteristlc. Accordy ingly'the characteristic marked3 -represents an operating condition more favorable for detection thanthat'marked 2; Qrepresents a more .favorable condition than 1, where thedetector is practically a simple conductor, without appreciable'.detecting properties.

The curvature and general form ofthe current-voltage characteristicsdepend first upon the composition of the crystalline substance, andsecond', upon the location and pressure of the contact point. Locationsupon the crystal surface, at which the current-voltage 4curve is highlyasymmetrical are commonly characterized as sensitive spots in thecrystal and can be located by connecting the crystal in series with atelephone receiver and with a source of modulated radio-frequencyvoltage and exploring the crystal while listening for the sound whichindicates, a detection of the modulated wave. It was by thisl methodthat the spots whose characteristics are shown at 2 and 3 were located.

The audio-frequency current which is passed by an asymmetrical conductorsuch Aas described is a function of the voltage and may be expressed as=f(e). The absolute strength of the signal is small and the device maybe polarized by means of an auxiliary steady E. M. F. so adjusted thatoperation takesplace about the point E on the characteristic curve 3 ofFig. l. Then expanding vf 25 in a power series about this point,denoting the difference between E and E@ by e, we have:

where the symbols f', f, f .indicate the first, second and thirdderivatives, respectively, ofi with respect to e, taken at the point E0.Now a radio telephone signal is impressed upon lthe detector and forsimplicity this signal will be taken as of the following generic type:

-mt) sin et (2)" Thev production ofa current of the type of themodulation function FU) is thus seen to The variation in the currentCAfl is distorted in the process of detection. This second-orderdetection, resulting in a squarelaw response and distortion of thesignal is an inherent result of lloperation upon an asymmetrical,continuously ycurved characteristic.' If the characteristic were notcontinuously curved the above power series vwould not be a legitimateexpansion of i as a function of e. In order that the audio current shallbe a faithful-copy of the original modulation of the signal 'thestimulus-response characteristic of the detection, or demodulationprocess mustv be linear; otherwise the various component vibrations inthe spectrum of the complex sound will not be dealt with in aproportionate fashion. As we have just shown, such a characteristic isimpossible of attainment using the property of asymmetrical conductivityof the detector as is the practice of the present day.

lt must be emphasized that although. a silicon crystal having acontinuously curved characteristic has been taken here as an example ofthe usual square-law detector, the same4 considerations and the samemathematical reasoning apply without modification to the ionic-tubedetector, wherein the detection of modulated signals is brought about bythe curvature (second and higher derivatives) lof the current-voltagecharacteristic of some conducting branch of the tube. It is a vfactfamiliar to most experimenters that the audio-frequency response of allionictube detectors is proportional to the square of the modulatedimpressed voltage over wide 10 ranges of operation.

It can be shown mathematically that the current-voltagecharacteristicgof ther ideal demodulator would consists of two straightlines meeting at an angle at some definite point, which point should bevused as the operating point. With a conductor of this eccentric naturethe audio-frequency currents owing as a result of the impression of amod- 'ulated radio-frequency voltage would be directly proportional tothe voltage amplitude. Experience has indicated, however, that noconductors of this ideal character exist and no person has been able tovproduce one by mechanical or electrical combinations. Continuouscurvature over a finite voltage range `has been exhibited by theconduction characteristics of all such combinations whichv do not followOhms law.

We have found that light metallic contacts upon a commercialferro-silicon alloy containing about to 80% silicon and about 20% to 30%iron, in the crystalline form in which this alloy comes from theelectric furnace, yield currentv voltage characteristics of which thoseshown in Fig. 2 at 1 and 2 are typical. T hese curves indicate that forimpressed voltages in one direction (arbitrarily taken as. positive uponthe diagram) the resistance is practically constant, giving a linearcurrent-voltage characteristic that the regions of curvature ofthecharacteristics are, in general, very limited, extending over notmore than 0.3 volt 'about the origin; and finally, that for voltages inthe reverse direction (negative) the resistance is practicallyconstantup to 1 volt. and for higher voltages the curvature isrelatively very small. We have found that contacts upon alloys of theabove-mentioned compositions yield curves having the nearest approach tovthe ideal shape of two intersecting straight lines. With ferro-siliconhaving more than iron the inclination to each other of the two branchesof the curves gradually increases without increasix the range ofcurvature, until at aboutv the point all contacts lose their rectifyling qualities and become simple straight-line conductors. Withferro-siliconscontaining 20% iron or less the reglon of curvature1ncreases and the, characteristics of sensltive contacts changegradually` with increasing' proportions of silicon into the quasi-'cubicform shown for the silicon. Itwill be understood that the foregoingstatements are based on the examination of a limited number of samples,and it is not excluded that ferrosilicon crystals having achemicalcomposition outside/of the preferred range indicated may befound well adaptedy for the purposes of this invention. Hence the abovestatements as to the preferred constitution of the lferro-silicon arenot to be regarded asnre'- strictive of the invention. It will also beunderstood that the ferro-silicon may contain minor quantities of metalsor elements other than iron and silicon. f

Itis rather diilicult, under these conditions, when the curves dependupon the location of contact as well as upon the nature of thefundamental substance, to isolate with certainty those qualities whichdepend only upon the composition. It should be noted however` thatalthough the curves for different contacts upon a substance of a. givencomposition may differ widely in extent and curvature, the general shapeor type of curves yfor all contacts on a given substance is the same;thus the general form of all curves for contacts upon commercially puresilicon (Fig. l) isv that of a cubic through the origin of co-`ordinates, one branch of which is displaced f or distorted from thetrue form representing a cubic equation, thus providing. a certainasymmetry about the origin. By investigating a large number of contactsupon different samples-of vthe ferro-silicon and upon the commerciallypurefsilicon we have determined conclusivelythat the form shown in Fig.2 is typical of sensitive spots upon the ferro-silicon, and that theform shown in Fig.

`1 is typical of sensitive spots upon the silicon.

and we have never found characteristics of any spots upon thelferro-silicon and the silicon which approachedeach other in formcomplished by using, in place of an iron or copper Contact point uponthe ferro-silicon, a point of iron pyritc (FeSz). -A splinter of thissubstance, when held in light contact with a sensitive spot on theferro-silicon, yields volt-ampere characteristics of which the one shownat 3 in Fig. 2 is typical. Owing to some superposition of the surfaceconducting qualities of the pyrite and the ferrosilicon, both branchesof the characteristic curve become so nearly straight lines thatit isimpossible tol detect any curvature at points removed from the mainbend, at the origin of coordinates.- We have found, however, that thisdesirable feature is compensated to some extent, from a practicalstandpoint by such a decrease in the relative-'inclination of :the twobranches of the curve that the rectifying sensitivity of the mostsensitive point that can be located with an iron-pyrite Contact pointsmaller than that obtainable with the simple iron or copper contactpoint.

Further experimental tests wlth radio-frequency impressed voltages have'shown that the slight departures of the characteristics of ourrectiliers from the ideal non-distorting curves of Fig. 3. Curvesshowing the direct current response as a function of the amplitude of animpressed radio frequency voltage are called stimulus-responsecharacteristics as distinguished from the voltage-currentcharacteristics which show the instantaneous rvalue of current whichflows through the rectifier as a function of the instantaneous impressedvoltage. Data for these curves were obtained by impressing upon therectifiers a measurable radio-frequency voltage, at a wave length ofapproximately 1900 meters (unmodulated) and measuring the resultingdirect component' of the resulting current flowing through therectifier.v The voltage amplitudes are plotted as abscissae, and therectified, or direct current as ordinates; curve 1 represents theresults for an iron point Acontact on avferro-sili-con crystal (-25%Fe), and curve 2, the results for a contact of iron pyrite upon asimilarcrystal. These curves indicate that the direct current response of ourrectifiers to unmodulated voltage is directly proportional 'to theamplitude of said voltage throughout a range of amplitude extending from0.3 volts to 3.5 volts. range, therefore, these rectiiiers behave asdistortionless deinodulators to modulated al- Within this .Y to thesquare of the amplitude of the radio s the audio-frequency response to amodulated radio-frequency voltage is directly proportional to theamplitude of the envelope of said voltage, and true demodulation, orfirst` power rectification results.

In order to illustrate more clearly the existence and boundaries of theregionin which we operate our rectifiers as distortionless demodulators,the diagram,.Fig. 4 is presented. The data for Fig. 4 were derived bycomputation from the experimental curves of Fig. 3. Suppose the radiosignal voltage to be demodulated is symmetrically modulated to 50%about-its mean value. Then if this signal voltage be so weak that itsmeanv value does not exceed 0.2 volt, its peak value does not pass outof the region of continuous curvature on the rectifier characteristic(Fig. 3)

' and the rectifier operates as a square-law detector, the' audioresponse being proportional input. This condition of operationis-represented by the region AB of the Fig. 4. If

the mean amplitude of the modulated input Vliesbetween 0.2 volt and 0.6volt, the input voltage crosses from the region of curvature on to thestraight Aportion of the rectifier characteristic in everyradio-frequency cycle, and the law of response of the rectifier cannotbe expressed as a single power of'the4 impressed voltage; this isindicated by the transition region shown on the diagram from B to C, inwhich the simple exponential relation:V I

1 @mam-:Aetnaxo breaks down, and the exponent k is indeterminate. Withsignal voltages whose mean amplitude exceeds 0.6 volt, however, theaudio output is proportional to the first power of the amplitude of themodulated input, and it is in thisv region that we operate ourrectifiers as distortionless demodulators, represented by C to D on thediagram; Of course the. extent of ,this range of linear responsedepends, for agiven rectifier characteristic, upon the degree ofmodulation of the incoming signal; this regionalways exists, however,and we have chosen the easev of 50% l'modulation merely to illustrateand to dee that the modulated amplitude of this signal voltage shall beso adjusted before being impressed upon the rectifier that it fallsWholly Within the above-defined range of'linear response of therectifier. For most rectifiers which fall within'the scope of ourinvention we contemplate amplification of such an ex- 'tent that thevoltage impressed upon the rectifier shall exceed l volt and may exceedten volts. We have discovered no definite upper limit to the range oflinear response with iron contacts on the ferro-silicon, and prefertherefore to employ .as high radio-frequency voltages as are consistentwith stable operarelate to the elimination ofthe effect of re-4 actionfrom plate to grid circuit in every tube, make possible the cascading oftubes in any number of stages, so that any desired amplification may beobtained.

The apparatus whichisemployed for securing the desired amplification anddistortionless detection of modulated radio fre-V quency waves maytake-various forms. As shown diagrammatically in Fig. 5, the deasfmodulator includes a multi-stage radio frequency amplifier 10 of anyappropriate construction from which the amplified signal wave is passedto a ferro-silicon rectifier 11 havin an iron or iron-pyrite contactpoint. Thej etected audio-frequency currents pass through the telephone`12 which is preferably shunted by a buv-pass condenser 13. It is yusually unnecessary to provide a bias voltage,

lbut when this is desired the audio-frequency circuit may include anauxiliaryyoltage divider 14.

.. In the preferred embodiment of the inven-` tion which is illustratedin Fig. 6, the demodulator includes a radio-frequencyinductive coupling15 between the' multi-stage radiofrequency amplifier 10 and Vtheferro-silicon rectifier .11E- The radio-frequency coupling preferablycomprises" a vario-transformer such as de scribedin thecopendingapplication of Ballantine, Serial No. 590,514, filed September25, 1922, and its purpose is to kee currents inadvertently rectified bythe amplifier out of the rectifier circuit. n The rectifier circuitincludes the secondary of the trans'- former 15, the rectifier 11, andtheprim'ary of an audio-frequency transformer 16. The primary of thetransformer 16 is preferably shunted by a condenser 17 and if desired a,voltage divider 14 may be included in the rectifier circuit. The audiofrequency apparatus 18 which is connected across the secondary of thetransformer 16 may comprise any suitable Aarrangement of audio-frequencyamplifying units, telephones, loud speaker, etc.

In laboratory tests with the circuit shown in Fig. 6, excellentdistortionless demodulation was obtained when the radio-frequencyamplifier comprises three UV 201A tubes coupled with vario-.transformersand thel transformer couplingl was also a variotransformer.

Although we have found that certain ferrosilicon compositions are welladapted for use in carrying out our method of dis'tortionlessdemodulation, particularly when such compositions are used wi'thiron-pyrite contacts, it will be understood that our invention is notlimited to rectifiers of this composition.

Our method of demodulation may be used with any rectifier havingconstant conductivcharacteristic has avery short curved por.

tion lying between the two ranges of substan-.

' tially uniform conductivity, i. e., the linear portions of thecharacteristic. WVhen the strength of the signals'is low oriwhen therectifier has such properties that the linear portions ofthecharacteristic are joined by a relatively longI curve, moreamplification will be necessary to satisfy the requirement that theenvelop of the modulated wave must fall within the range of linearresponse.

We claim: l. Method of operating a radio'receiving system suitable -forthe reception of modu- -vlated carrier-wave signals and ofthe typeincluding a de'modulator characterized by the fact that over asubstantial range of impressed voltages the relation between impressedvoltage' and output current'is substantially linear, which comprisesamplifying received signal voltages to values within said range oflinear response, and impressing aid amplified voltages upon saiddemoduator.

2. Method of operating a radio receiving system suitable for thereception of carrier- 'wave signals and of the type including ademodulator characterized by the fact that over a substantial range ofimpressed voltages in excess of approximately 1.0 volt the relationbetween impressed voltage and output current n is substantially linear,which comprises amplifying received signalvoltages to valuessubstantially in excess of 1.0 volt, and impressing said amplifiedvoltages upon said demodulator. f

the reception of modulated carrier-wave sigg na-ls and comprising, incombination, a demodulator characterized by the fact that over asubstantial range of impressed voltages the relationbetween impressedvoltage and output current is substantially linear; means for amplifyingreceived signal voltages to values within said range of linea-rresponse; and means for impressing said amplified voltages of saidvalues upon said demodulator.

4. A radio receiving system suitable for the reception of modulatedcarrier-wave signals, and comprising, in combination, a demodulatorcharacterized by the fact that over a substantial range of impressedvoltages in excess of approximately 1.0 volt, the relation betweenimpressed voltage .and output lcur-` rent is. substantially linear;means for amplifyingreceived signalv voltages to values substantially inexcess of l volt; and means for impressing said amplified voltages ofsaid values upon said demodulator.

3. A radio receiving system suitable forr 5. A radio receiving systemsuitable for the reception of modulated carrier-wave signals andcomprising, in combination, a demodulator characterized by the fact thatover a substantial range of impressed positive voltages the relationbetween lapplied voltage and output current is substantially linear,over a substantial range of impressed negative voltages the relationbetween applied voltage and output current is substantially linear, andover'a small intermediate range of impressed positive and negativevoltages the current-voltage characteristic is curved, the slopes ofsaid linear portions of the current-voltage .characteristic'beingfdifferent so that said linear portions are mutually inclined;means for .amplifying received signal voltages to values substantiallywithin the region represented by said linear branches; and means forimpressing said .amplified voltages of said values upon saiddemodulator. Y

n 6. A radio receiving system suitable for th reception of modulatedcarrier-wave signals, and comprising, in combinatioma demodulatorcharacterized by .the fact that over a substantial range of impressedpositive voltage in excess of approximately 1.0 volt the relationbetween applied voltage and output current is substantially linear, overa substantial range of impressed negative voltages in excess ofapproximately 1.0 volt the relation between applied voltage and outputcurrent is substantially linear, and over a small intermediate range ofimpressed positive and negative-voltages lying between approxi#v curved,the slopes of sald linear portionsof the current-voltage characteristicbeing diff ferent so that said linear portions are mutually inclined;means for amplifying received 4signal voltages to values substantiallyin excess of l volt; and means for impressing said amplified voltages ofsaid values upon said 'demodulator 7. A radio receiving system suitablefor f the reception of modulated carrier-wave signals and comprising, incombination, a detector crystal composed of ferro-silicon, andcharacterized by the fact that its voltagecurrent characteristiccomprises two mutu- 10 ally inclined ysubstantially linear branchesjoined by a relatively short curved portion; a radio frequency amplifierfor amplifying incoming signals to Values substantially with-V in theregion represented by said linear i5 branches; and means for'impressing;said amplified voltages of saidvalues upon said detector crystal. v 8.In combination, a detector comprising 'ferro-silicon and characterizedby the fact that its voltage-current characteristic com-- prises twovmutually inclined substantially linear branches joined by a shortcurved portion; and means for so adjusting the voltage of the incomingradio frequency waves that it will substantially exceed that criticalvalue above which the rectified current is lineally proportional to theimpressed radio frequeney voltage.

9. A detector for radio frequency oscillations, composed of an alloy ofiron and silicon between the limits of 20 percent iron-8O percentsilicon, and 30 percent iron-#7() per- .l

cent silicon, saidy alloy being in crystalline form, and characterizedby a voltage-current 36 characteristic curve having two mutuallyinclined substantially linear branches.

10; Ina demodulator, a rectifying device comprising a crystal offerro-silicon and an iron-pyrite contact. l

40 11. The methodof operatin a radio receiver including a detector, wich includes adjusting said detector forits maximum lrectification andapplying tosaid detector a modulated carrier wave of such amplitude asto Iproduce a substantially linear ratio between the rectified responseand the percentage modulation of said carrier wave. f

In testimony whereof we afiix our signatures.

5o' i STUART BALLANTINE.

` LEWIS M. HULL. f

